The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
The present invention relates to methods, apparatuses and compositions pertaining to infrared radiation detection, more particularly to the photon detection of infrared radiation such as associated with thermal emissions.
The electromagnetic spectrum has conventionally been divided into approximate regions according to wavelength. The visible region, approximately in the range between 400 nm and 700 nm, corresponds to electromagnetic radiation to which the human eye is sensitive (visible light). The regions of successively shorter wavelengths than the visible region are ultraviolet, x-rays and gamma rays. The regions of successively longer wavelengths than the visible region are the near infrared, infrared and radio waves.
The near infrared region (NJR) approximately encompasses the 700 nm to 1 xcexcm range. The infrared region approximately encompasses the 1 xcexcm to 100 xcexcm range. The infrared region is approximately subdivided into: short wave infrared (SWIR), having wavelengths approximately between 1 xcexcm and 3 xcexcm; midwave infrared (MWIR), having wavelengths approximately between 3 xcexcm and 5 xcexcm; and, long wave infrared (LWIR), having wavelengths longer than about 8 xcexcm and up to about 100 xcexcm. The region between MWIR and LWIR is conventionally disregarded due to strong atmospheric absorption. Radio waves have wavelengths longer than about 100 xcexcm.
An electromagnetic radiation detector (also referred to as a photodetector, or an electromagnetic radiation sensor, or an electro-optic detector or sensor) is a device which absorbs electromagnetic radiation and gives rise to an electrical output signal that, generally speaking, is proportional to the irradiance (the intensity of the incident electromagnetic radiation). Depending on the type of detector, the output signal will be either a voltage or a current. In comparison with thermal detectors, photon detectors are characterized by a higher speed of response. Current semiconductor photon detectors having long wavelength limits in the ultraviolet, visible or near infrared (up to 2-3 xcexcm) operate uncooled at room temperature (300 degrees kelvin, or 300 K). At longer, midwavelength infrared (MWIR) limits (up to 4-5 xcexcm), cooling to dry ice temperature (195 K) is required. For detectors operating in the long wavelength infrared (LWIR) 8-12 xcexcm range, cooling to liquid nitrogen temperature (77 K) is essential.
Because all bodies at temperatures greater than absolute zero radiate in the infrared radiation region, infrared radiation detection has been of importance in military applications. By employing infrared radiation detection (e.g., via infrared xe2x80x9cseekersxe2x80x9d), warm targets can be detected in the dark by virtue of their own infrared radiation, thus obviating the need to illuminate such targets in order to render them visible. Warm bodies emit infrared radiation, and bodies which absorb infrared radiation are warmed. It is incorrect, however, to call infrared radiation xe2x80x9cheat radiation,xe2x80x9d because the radiation itself is not xe2x80x9cheat.xe2x80x9d
Higher operating temperature has been a goal of infrared detection development for the last few decades. Direct bandgap alloy semiconductor-materials such as HgCdTe replaced extrinsic germanium and silicon devices for LWIR applications because they could operate under ambient background flux conditions at 80 K. It has been thought that, theoretically at least, if 12 xcexcm detectors could operate at 80 K, then 5 xcexcm detector operation at 180 K should be possible.
At MWIR wavelengths, InSb has remained the infrared detector of choice for many applications. InSb has a spectral cutoff at 5.5 xcexcm at 80 K, but its bandgap of 0.22 eV narrows as the temperature increases, extending its spectral response into the water vapor band between 5.5 and 7.5 xcexcm, and also resulting in a very rapid increase in thermally generated noise. InSb detectors cannot operate effectively above about 145 K, and are seldom used above 100 K. Within the past decade HgCdTe and InAsSb photoconductive and photodiode technology has matured in the MWIR spectral band so that operation at 180 K, using thermoelectric coolers as well as mechanical coolers at 120 K, has been possible. The spectral response of thermoelectric cooled HgCdTe detector has a cutoff of 5 xcexcm at 180 K.
State-of-the-art performance is often desired in the realm of infrared radiation detection; in general; in order to be optimal, infrared radiation detection requires use of very high quality material. InSb and HgCdTe are both very mature for use in the NWIR spectral region. InSb is an equally sensitive alternative to HgCdTe for MWIR applications. InSb is easier to produce at high quality than HgCdTe, and has found a niche in the marketplace as a cost-effective alternative for high-sensitivity MWIR applications that require good, corrected uniformity. See, e.g., J. L. Miller, Principles of Infrared Technologyxe2x80x94A Practical Guide to the State-of-the-Art, Van Nostrand Reinhold, John Wiley and Sons, Inc., New York, 1994, incorporated herein by reference; see, especially, pages 370-431.
The availability of photovoltaic HgCdTe and InSb infrared image detectors continues to expand rapidly as the technology has matured and entered a transition to production for both commercial and military applications. Detector costs for staring array formats, however, continue to limit the market demand. Although a seeker containing these arrays represents a small percentage of the weight of a missile system, it represents a large percentage of the costxe2x80x94up to 50% or more; see, e.g., aforementioned book by J. L. Miller entitled Principles of Infrared Technologyxe2x80x94A Practical Guide to the State-of-the-Art. Although a missile seeker could, therefore, conceivably be produced for only tens of thousands of dollars, in reality missile seeker development is still expensive and can run from tens to hundreds of millions of dollars.
In view of the foregoing, it is an object of the present invention to provide method and apparatus for effectuating midwavelength infrared radiation photon detection.
It is another object of the present invention to provide a high caliber composition for optimally effectuating midwavelength infrared (MWIR) radiation photon detection.
It is a further object of this invention to provide apparatus, including such high caliber composition, for optimally effectuating midwavelength infrared radiation photon detection.
It is another object of this invention to effectuate midwavelength infrared radiation detection at relatively high temperatures (e.g., room temperature), so that such detection does not require cooling or only requires relatively moderate cooling.
A further object of this invention is to effectuate midwavelength infrared radiation photon detection of both polarized and unpolarized radiation.
Another object of this invention is to effectuate midwavelength infrared radiation photon detection efficiently and economically.
According to many embodiments of the present invention, these objects are achieved by providing fibers of Niobium Trisulfide (NbS3) and an insulative substrate. The NbS3 fibers form a single layer of approximately parallel sensing segments resting on an electrically insulating quartz (or other insulating material) substrate. According to some embodiments of this invention, an assembly includes NbS3 fibers (along with their corresponding insulative substrates) which are arranged in four types of fiber orientations (viz., 0 degrees, 45 degrees, 90 degrees and 135 degrees) as part of an extended focal plane array; this inventive assembly permits the detection of polarized and unpolarized infrared light (radiation); that is, the array permits infrared detection of plural polarizations of infrared radiation.
The present invention provides an infrared electro-optic charge-density-wave conducting material, viz., Niobium Trisulfide (NbS3), in crystalline fibrous form, for use in association with midwavelength infrared radiation photon detection. The present invention also provides a device for infrared radiation detection, the device comprising NbS3 and a substrate. For use in association with photon detection of midwavelength infrared radiation, the inventive combination includes an approximately parallel, fibrous configuration of NbS3 situated atop an insulative (nonconductive) substrate. According to typical such inventive embodiments, charge-density-wave fibers of NbS3 are provided for an uncooled detector in the 3-5 xcexcm infrared wavelength range.
This invention provides a midwavelength infrared radiation photon detector and a methodology for midwavelength infrared radiation photon detection. A typical infrared detector according to this invention comprises the combination of plural Niobium Trisulfide crystalline fibers and an insulative substrate. The insulative substrate has an approximately flat substrative surface. The Niobium Trisulfide fibers are adjoinedly disposed in approximately parallel fashion on the substrative surface. The detector is positionable relative to incident infrared radiation so that the substrative surface is approximately orthogonally facing the incident infrared radiation. A typical infrared detection method according to this invention comprises positioning such an inventive device relative to incident infrared radiation so that the substrative surface is approximately orthogonally facing the incident infrared radiation.
According to many inventive embodiments, an uncooled polarized radiation photon detector is intended for utilization in focal plane arrays for the purpose of sensing polarized thermal emissions. Frequently according to such embodiments, this invention provides a detector with thin fibers of NbS3 lying in parallel on a rectangular noninsulative (e.g., quartz) substrate, and with thin strips of electrically conductive metallic (e.g. tin) films coupling the fibers to leads for connection to external circuitry.
According to many embodiments of the present invention, the inventive midwavelength infrared detector includes pre-grown parallel thin, flat, narrow crystalline fibers of NbS3 charge-density-wave conductors resting flatwise on a rectangular quartz substrate on which thin strips of evaporated tin films electrically couple the ends of the fibers to leads for connection to external circuitry. The inventive NbS3 electrical conductors have an energy gap of about 0.3 eV for temperatures below 355 K, enabling sensing in the 3-5 xcexcm wavelength range. The flat fibers sense the radiation of an emitting object through the excitation of unpaired charge carriers by means of infrared photons breaking up paired charges. The inventors have made a prototypical embodiment of the present invention""s infrared detector which demonstrates the efficacy thereof.
The present invention can be used individually to detect the presence of infrared radiation, or can be used in arrays to provide additional information such as spatial resolution (e.g., for an imager or spectrometer). The inventive arrays can include like or unlike inventive detectors. A particularly noteworthy latter kind of inventive array comprises plural detectors characterized by varying NbS3 fiber orientations. In this regard, a preferred embodiment of the present invention provides a combination of plural midwavelength infrared detectors, such combination including at least one set of four detectors, wherein the detectors of a given set have parallel fibers oriented at 0xc2x0, 45xc2x0, 90xc2x0 and 135xc2x0 for use in a focal plane array. In other words, the combination of plural detectors includes: at least one square region having fibers oriented at 0xc2x0; at least one square region having fibers oriented at 45xc2x0; at least one square region having fibers oriented at 90xc2x0; and, at least one square region having fibers oriented at 135xc2x0. According to many such inventive embodiments, a plurality of the inventive uncooled detectors, including at least one set of four of the inventive uncooled detectors wherein the parallel fibers are oriented at the four different orientations (0 degrees, 45 degrees, 90 degrees and 135 degrees), enables the development of a focal plane array for detection not only of polarized infrared radiation, but also of unpolarized infrared radiation. Such arrays may be used in lightweight weapon seekers which are suitable for advanced target recognition, decoy discrimination and clutter rejection.
By way of explanation, normally the waves of a beam (ray) of radiation (light) are disorderly; that is, although each wave vibrates in a direction perpendicular to its path, there is no favored orientation or direction of such vibrations. Such light beams are referred to as xe2x80x9cunpolarized.xe2x80x9d However, when all of the waves in a light beam vibrate in parallel planes and in the same perpendicular direction, such light beams are referred to as xe2x80x9cpolarizedxe2x80x9d (e.g., xe2x80x9clinearly polarized,xe2x80x9d as distinguished from xe2x80x9ccircularly polarizedxe2x80x9d or xe2x80x9celliptically polarizedxe2x80x9d). These well-known scientific principles concerning polarized and unpolarized electromagnetic radiation are applicable to the present invention. A single inventive infrared detector will normally be photosensitive only to those components of infrared radiation beams which are polarized in comportment with the particular configuration of its parallel Niobium Trisulfide fibers; in other words, that detector will sense only the portions of the infrared radiation which are polarized insofar as being characterized by waves which vibrate in planes which are parallel to each other and in identical perpendicular directions which are parallel to the parallelly arranged Niobium Trisulfide fibers. On the other hand, an array of infrared detectors according to this invention will include at least four detectors corresponding to at least four orientations of the parallelly distributed Niobium Trisulfide fibers. Each detector of the array will be photosensitive to infrared radiation which is polarized in agreement with the Niobium Trisulfide fiber parallelness specific thereto. However, with the assistance of apparatus such as including a processor, information can be obtained according to this invention as to other, xe2x80x9cin-betweenxe2x80x9d directions of infrared waves, based on infrared wave data pertaining to the actual fibrous orientations. In fact, a complete or nearly complete representation of the xe2x80x9cunpolarizedxe2x80x9d infrared light beam can be achieved in accordance with inventive practice of detector arrays.
Advantageously, the present invention can afford moderately cooled or uncooled infrared detection. The present invention succeeds in reducing or eliminating the requirement of cooling. Hence, infrared seeker packaging will be more lightweight and compact with lower power consumption. Furthermore, seeker packaging according to this invention will be much less costly, because NbS3xe2x80x94a detector material other than a semiconductor materialxe2x80x94is implemented. Moreover, system reliability will be improved.
The present invention features, inter alia, the nonsemiconducting charge-density-wave crystalline fibrous material NbS3, which possesses an energy gap of about 0.3 eV for temperatures below 355 K. Although the crystalline fibrous material NbS3 is known generally, it has never been used or known to be useful in any infrared detection applications. In the context of any manner of infrared radiation detection (including but not limited to midwavelength infrared radiation detection), the present invention""s mere use of NbS3 in any photosensitive capacity (e.g., as a photoconductive material), in and of itself, represents a significant, unique and previously unknown improvement in the art. The energy gap in InSb is smaller at 0.18 eV at 300 K, whereas the gap can be tailored in the ternary alloys HgCdTe and InAsSb for use at longer wavelengths. The much more robust energy gap in NbS3 greatly reduces the dark current noise, which rapidly increases above 80 K in the semiconductors, making the semiconductor materials practically useless for IR detection much above 80 K.
HgCdTe, InAsSb and InSb can detect through either of two processes, viz., the photoconductive semiconductor mode of varying conduction with illumination intensity, or the photovoltaic (photodiode) mode wherein the voltage across the detector is generated from the incident radiation. In the photoconductive mode, these detectors are characterized by moderate to high detectivity and relatively low frequency response. On the other hand, similar photoexcitation properties, but faster response times are observed in thin-film high temperature superconductors; see R. Sobolewski, xe2x80x9cUltrafast Dynamics of Nonequilibrium Quasiparticles in High Temperature Superconductors,xe2x80x9d in Proceedings of SPIE, Vol. 3481, Jul. 20-24, 1998, pp 480-491, incorporated herein by reference. These superconducting materials are being considered for detection, since their critical temperatures TC greater than 90 K exceed the best temperatures for cooling of InSb and HgCdTe.
Other studies of these superconducting materials have indicated, however, that the photoeffect must occur at lower temperatures in the 10 K-30 K range; see M. G. Forrester and J. Talvacchio, xe2x80x9cPhoton Detection by High Temperature Superconducting Films: Fundamental Limits,xe2x80x9d Physica C, Vol. 162-164, pp 391-392 (1989), incorporated herein by reference. At higher temperatures, the generation-recombination (g-r) noise would limit the detectors. That is, in the absence of incident radiation, there would be an equilibrium density of quasiparticles due to thermal excitation. Fluctuations in their generation rate from breakup of Cooper pairs, and their recombination into Cooper pairs, would constitute noise in the readout.
Generally speaking, charge density wave (CDW) materials are analogous to superconductive materials, particularly insofar as being characterized by a kind of xe2x80x9ccondensationxe2x80x9d effect wherein under certain circumstances electrons tend to behave collectively (but do not behave so in the presence of a magnetic field). In this regard, the charge density wave material NbS3 has certain properties similar to those of superconductors. In the CDW material NbS3, the much larger energy gap of 0.3 eV leads to a much smaller thermal generation rate and a lower g-r noise level. In fact, the g-r noise for NbS3 is smaller than the level in any of the semiconductors with smaller energy gaps of  less than 0.22 eV. Thus, since the spectral detectivity or specific sensitivity is inversely proportional to the square root of the g-r noise, NbS3 is the superior material of choice, even at temperatures colder than 300 K.
NbS3 is only one member of a series of CDW materials. See the following references, each of which is incorporated herein by reference: P. J. Sarman, R. D. Bardo and R. Chen, xe2x80x9cCharge Density Wave Materials,xe2x80x9d in FY98 NSWC Carderock Division Research Digest, Naval Surface Warfare Center, Carderock Division, CARDEROCKDIV-99/CT01, Mar. 1999, pp 73-75; P. J. Sarman, R. D. Bardo and R. Chen, xe2x80x9cCharge Density Wave Materials,xe2x80x9d in FY99 NSWC Carderock Division Research Digest, Naval Sea Systems Command, Naval Surface Warfare Center, Carderock Division, CARDEROCKDIV-00/CT01, March 2000, pp 48-50. Also notable is R. D. Bardo, P. Sarman and R. E. Thorne, xe2x80x9cInfrared (2.5 xcexcm to 13.5 xcexcm) Reflectance Measurements and Calculations of the Peierls Gap Energy in NbSe3,xe2x80x9d accepted for publication in Phys. Rev. B. Nevertheless, NbS3 has, by far, the highest critical temperature of 355 K.
Moreover, single, high-quality fibrous NbS3 crystals are easily and inexpensively grown in a three-zone infrared furnace at temperatures of 650xc2x0 C. NbS3 crystals of any length up to 1 cm may be routinely grown, enabling the design of detectors up to 1 cm2. As illustrated in FIG. 1 herein, the reflectance measurements performed by the present inventors (also the authors of the aforementioned articles by Sarman and Bardo, each article entitled xe2x80x9cCharge Density Wave Martialsxe2x80x9d) on these NbS3 crystals confirm their high quality and the existence of the energy gap in the 3-5 xcexcm wavelength range.